1. Field of the Invention
This invention relates to data transmission systems in which a transmitting station sends sequentially numbered information frames to a receiving station, and, more particularly, to a method and a system for controlling the retransmission by the sending station of incorrect information frames. This invention is especially useful in those data transmission systems which use a control procedure of the type known as the HDLC (High-Level Data Link Control) procedure.
2. Description of the Prior Art
In data transmission systems using a HDLC-type of control procedure, each station transmits information organized in a specific format called an information frame, and all frames so transmitted are sequentially numbered. The receiving station checks the incoming frames for transmission errors and requests the sending station to retransmit incorrect frames through the use of a Reject (REJ) command or a Selective Reject (SREJ) command. The REJ command contains a frame number and is used to request the retransmission of all information frames starting at that number. The SREJ command is similar to the REJ command, but is only used to request the retransmission of the specific frame whose number appears in the SREJ command. Both commands also serve to acknowledge the receipt of all frames preceding the one whose number appears therein.
However, in data transmission systems operating at megabit rates and exhibiting round-trip propagation delays of the order of 500 ms and more, as in satellite transmission systems, the use of the REJ and SREJ commands presents some problems, as the following example will show.
Let us assume that a data link between Station A and Station B exhibits a round-trip propagation delay equivalent to the transmission of 39 frames, that Station A sequentially transmits frames numbered 00, 01, 02, etc., and that frames 03 and 06 are not correctly received by Station B. Upon receipt of frame 04, Station B will discard frame 04 and all subsequent frames and will transmit a REJ command requesting reinitiation of the sequential transmission of frames starting at frame 03, which command also acknowledges the receipt of frames 02 and below. However, because of the propagation delay, frames 00 to 43 have already been transmitted by Station A before the REJ command is received, with the result that the time spent in transmitting frames 04 through 43 is lost.
In the foregoing example, a SREJ could have been used to request the retransmission of frame 03. However, since the SREJ command, like the REJ command, acknowledges the receipt of all frames preceding the one whose retransmission has been requested, a request for the retransmission of frame 06 cannot be initiated until frame 03 has been correctly received. Consequently, only one SREJ command can be sent per round-trip propagation delay.
The foregoing shows that, in transmission systems exhibiting significant round-trip propagation delay, the use of REJ or SREJ commands will cause major degradation in throughput, assuming average transmission error rates.